Heat oven to 425° F. On a rimmed baking sheet, toss the potatoes, onion, 2 tablespoons of the oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Roast until tender, 20 to 25 minutes.
Meanwhile, heat the remaining tablespoon of oil in a skillet over medium-high heat. Season the chicken with 1/2 tea...